1. Field of Invention
This invention relates to a lossless compression technique and an apparatus in which an image treated with the Roberts method is predictively encoded and decoded.
2. Description of Related Art
There is a class of image compression techniques where the number of bits per pixel of an image is reduced and then the bit reduced image is compressed using lossless techniques. This approach is attractive because the first step significantly reduces the data to be compressed and, therefore, even a moderate compression ratio in the compression step yields a rather high overall compression ratio. For example, if an image is first reduced from 8 bits per pixel to 2 bits per pixel, which is a reduction factor of 4, and a lossless compression technique is applied that provides a compression ratio of 5, then the overall compression ratio is 20. Additionally, if the bit reduction step is performed with the final image quality in mind, then it is possible to achieve a more favorable quality/cost tradeoff.
The bit reduction techniques that are commonly used are thresholding, halftoning and error diffusion. Thresholding is generally good for text and halftoning is generally good for continuous tone images. However, neither thresholding nor half toning is suitable for all image types. Error diffusion is a good general purpose method for processing all types of images and is, therefore, widely used.
The system and method of this invention uses the Roberts method for the bit reduction method and then predictively encodes the coarsely quantized image. In the Roberts method, a psuedo-random noise is added to an image and the result is coarsely quantized. As an example, pseudo-random noise is added to an 8 bit per pixel image that is coarsely quantized to 2 bits per pixel. This quantization reduces the amount of data by a factor of 4. The knowledge of the generated pseudo-random numbers is then utilized to improve prediction and therefore improve data compression.